a) Field of the Invention
The invention is directed to a miniaturized optically imaging system with high lateral and axial resolution, particularly for endoscopic applications, which uses at least one GRIN lens system with a radial refractive index profile which generates an optical imaging of near-surface areas of an object in an image plane for an optical transmission system.
b) Description of the Related Art
Miniature optics play an important role for microscopy imaging, particularly in endomicroscopy. Endomicroscopic examination instruments are characterized by an optical image recording head which is small in diameter (a few millimeters) and in which the image must be transmitted over very long distances compared to the diameter.
Miniature optics of this kind image the surface of the object at high resolution in the micrometer range. The image is generated in transmitted light mode, in fluorescence microscopy, reflection microscopy, confocal microscopy or by scanning multiphoton fluorescence excitation.
In applications of this type, gradient-index (GRIN) lenses are used for the construction of miniature optics based on experience because they have a remarkably good imaging quality with typical small diameters between 0.2 and 2.0 mm and can be favorably integrated in small lens systems because of their generally plane optical surfaces. In GRIN lenses, the optical image-generating action is based on building a refractive index profile in the lens medium and, therefore, the refraction between two media (e.g., between air and glass) at curved surfaces is not necessarily needed for image formation. Therefore, miniature optics are comparatively simple to assemble. Miniaturized optics of this kind are known, for example, for endoscopic applications from EP 1 283 413 B1 and for laser scanning microscopes from DE 100 15 455 A1.
The solutions mentioned above are disadvantageous with respect to the imaging quality that can be achieved, which is clearly deficient primarily because of a low photon efficiency, a limited lateral and axial resolving capacity (due to the known relatively low numerical aperture of GRIN lenses of about 0.55) and the chromatic aberration of the GRIN lenses.